This invention relates to a process for the heat treatment of fine-grained material, particularly cement raw mix, which is preheated with the hot waste gases of a revolving tubular kiln in a multistage cyclone preheater, further heated, particularly calcined, by additional solid fuel, and roasted to completion in the revolving tubular kiln.
In order to relieve the revolving tubular kiln of thermal work and hence reduce its dimensions, it is known that additional fuel may be introduced into the lowermost stage of the preheater so that the material already preheated by the waste kiln gases is further heated (in the particular case of cement raw mix largely calcined) before it subsequently enters the revolving tubular kiln (see German Auslegeschrift No. 2,324,565).
Only a relatively short time is available for transferring the thermal energy present in this additional fuel to the material. Accordingly, it is important in the known process to mix the fuel and the material as quickly and as uniformly as possible in the lowermost stage of the preheater and then to obtain virtually complete combustion of the fuel on or in the immediate vicinity of the material particles.
Where liquid or gaseous fuel is used, no significant problems are involved in meeting the abovementioned requirements. However, this is not the case where solid fuel is used. In order to obtain sufficiently rapid and uniform combustion of the fuel in the lowermost stage of the preheater, the solid fuel has to be dried and very finely ground beforehand which involves considerable costs.
Further difficulties arise where low-quality fuel having a high content of incombustible constituents is used. When fuel of this type is introduced into the lowermost stage of the preheater, a considerable amount of incombustible, cold substances enters the lowermost stage of the preheater as part of the fuel/material mixture and then immediately afterwards the revolving tubular kiln, which seriously impairs the necessary intense calcination of the material before entry into the revolving tubular kiln.